The present invention relates to a method for decomposing bromic acid, that is, bromate ions contained in a liquid, using a photocatalyst, and an apparatus for the decomposition.
Kurokawa et al. (1986) JNCl, Vol. 77, No. 4, pp. 977-982 describes carcinogenicity of potassium bromate. Bromate ion (BrO331 ) can be generated by dissolving potassium bromate in water. Bromate ion can also be produced as a by-product by oxidizing bromide ion (Brxe2x88x92) dissolved in water, in the ozonization or accelerated oxidation treatment of drinking water. Bromate ion is classified by IARC (International Agency for Research on Cancer) as. Group 2B of having the possibility of carcinogenicity. In Japan, ozonization has increasingly been used in purification of drinking water in order to eliminate bad smell of drinking water or to reduce the amount of trilialomethane generated as a by-product by disinfection with chlorine. Thus, much attention has been drawn to bromate ion due to its carcinogenicity. The permissible bromate ion concentration of drinking water was set to 25 xcexcg/L by WHO. U.S. Environmental Protection Agency has proposed a permissible bromate ion concentration of 10 xcexcg/L at the first stage of Disinfectant/Disinfection By-product Rule (D/DBPrule) and may propose a stricter concentration at the second stage of D/DBPrule.
Asami et al. (1996) xe2x80x9cMizu Kankyo Gakkai-shixe2x80x9d, Vol. 19, No. 11, pp. 930-936 describes bromate ion formation inhibition by coexisting organic matters in ozonation process. Miyata et al. (1997) xe2x80x9cSuido Kyokai Zasshixe2x80x9d, Vol. 66, No. 3, pp. 16-25 describes the removal of bromate ion by particulate activated carbon. Particulate activated carbon, however, may become deteriorated in the removal of bromate ion, as the activated carbon adsorbs thereon dissolved organic matter and the like. The deteriorated activated carbon may require the replacement with new one or reactivation. Furthermore, it has been proposed to suppress the formation of bromate ion by strictly controlling the amount of ozone to be injected into drinking water.
The amount of bromate ion generated by ozonization is known to be substantially in proportion to CT value that is the product of the concentration (C)of dissolved ozone and the ozonization time (T). On the other hand, the degree of disinfection is substantially in proportion to CT value. Thus, CT value is required to be at least a predetermined minimum value in order to have a sufficient disinfection. FIG. 16 shows the change of bromic ion concentration with ozone injection rate by black circles and the change of C*T10 with ozone injection rate by white circles, for destroying Giardia. As shown in FIG. 16, CT value becomes sufficient to destroy Giardia when the ozone injection rate is at least 1.8 mg/L. Under this condition, the bromate ion concentration becomes about 3 xcexcg/L. It may be difficult to avoid the generation of a certain amount of bromate ion in order to sufficiently disinfect drinking water.
It is therefore an object of the present invention to provide a method for efficiently and stably decomposing bromate ions contained in a liquid by a photocatalytic reaction.
It is another object of the present invention to provide an apparatus therefor.
According to the present invention, there is provided a method for decomposing bromate ions contained in a liquid. This method comprises bringing the liquid into contact with a photocatalyst; and irradiating the photocatalyst with a light ray having an energy that is not lower than that of a band gap of the photocatalyst, thereby generating a photocatalytic reaction to decompose the bromate ions.
According to the present invention, there is provided an apparatus for decomposing bromate ions contained in a liquid. This apparatus comprises a first section for generating therein a photocatalytic reaction to decompose the bromate ions; a photocatalyst adapted to be brought into contact with the liquid in the first section; and a light source for irradiating the photocatalyst with the light ray such that the photocatalytic reaction is generated in the first section when the photocatalyst is in contact with the liquid.